Sound energy emitted by sound sources, such as centrifugal or axial flow fans, is often measured through use of a microphone but the measurement is often complicated because of the presence of flow noise.
Flow noise can be generated by two sources. First, there is self-noise caused by the presence of the microphone unit itself. Microphone nose cones and wind screens can act to reduce this self-noise. Second, there is flow noise associated with the turbulent pressure fluctuations caused by the fan. Probe tubes have been proposed in an attempt to desensitize the microphones to turbulent flow pressure fluctuations. In general, the microphone probes, as used in the past, have consisted of an elongated tube having a tapered end facing upstream of the direction of sound propagation and containing a microphone in the opposite end. The tube includes a plurality of small spaced openings, or alternately, an elongated slit, which is covered with an acoustically resistive material, such as cloth or felt. Screens of this type are designed to distinguish between turbulent pressure fluctuations, which have a low propagation velocity, and sound pressure fluctuations, which have a much higher propagation velocity. Pressure fluctuations at locations on the tube cause pressure waves to propagate inside of the tube with the propagation velocity being nearly equal to the speed of sound. If the pressure fluctuations are sound waves, the pressure waves from all locations will add constructively and create a high sound pressure at the microphone because the propagation velocity of the sound inside and outside of the tube is nearly the same.
On the other hand, if the pressure fluctuations are caused by turbulent flow around the tube, the pressure waves inside the tube will add destructively, because the propagation velocity inside the tube is much higher than the propagation velocity of the turbulence outside the tube, and thus only a small pressure will therefore be created at the microphone diaphragm.
However, problems have arisen with the use of conventional microphone probes. The ability of the probe to screen turbulent flow pressure fluctuations is affected by the tautness of the acoustic material which is disposed across the openings or slit in the tube. In the past, the material has been applied to the tube as a helical or spiral winding and in practice certain portions of the material may be more tightly wound than others, thereby providing a non-uniform acoustic resistance and adversely affecting performance of the instrument. Alternatively, the material has simply been held against the openings with no control over the acoustic resistance.